Problems with > ~50 uploads in a single request

I'm trying to upload files from a Java applet through mod_porter/Passenger 2.2.15/Apache 2.2.3/Ruby 1.8.7/Rails 2.3.5.

Everything works as expected when I upload 5, 10, or even 50 files at a time. If I try a batch of ~60, however, I only receive the params hashes (content_type, path, etc.) for the first ~55 files. I've reviewed the mod_porter debug log, and it seems that the files are all being recognized and processed by mod_porter. Note I am able to upload an arbitrary number of files to a PHP backend using the same applet, so I believe the applet is forming the POST correctly.

Can you help determine what's causing this? It occurs each attempt, regardless of which files I upload and their sizes (the cutoff point varies, though, depending on which files I upload).

Relevant log data:
mod_porter Rails plugin expects all 62 files:
Processing ["FileBody_0", "FileBody_1", "FileBody_2", "FileBody_3", "FileBody_4", "FileBody_5", "FileBody_6", "FileBody_7", "FileBody_8", "FileBody_9", "FileBody_10", "FileBody_11", "FileBody_12", "FileBody_13", "FileBody_14", "FileBody_15", "FileBody_16", "FileBody_17", "FileBody_18", "FileBody_19", "FileBody_20", "FileBody_21", "FileBody_22", "FileBody_23", "FileBody_24", "FileBody_25", "FileBody_26", "FileBody_27", "FileBody_28", "FileBody_29", "FileBody_30", "FileBody_31", "FileBody_32", "FileBody_33", "FileBody_34", "FileBody_35", "FileBody_36", "FileBody_37", "FileBody_38", "FileBody_39", "FileBody_40", "FileBody_41", "FileBody_42", "FileBody_43", "FileBody_44", "FileBody_45", "FileBody_46", "FileBody_47", "FileBody_48", "FileBody_49", "FileBody_50", "FileBody_51", "FileBody_52", "FileBody_53", "FileBody_54", "FileBody_55", "FileBody_56", "FileBody_57", "FileBody_58", "FileBody_59", "FileBody_60", "FileBody_61"]

mod_porter log shows processing of all files (up to an including 61):
[PORTER_LOG] Handling Upload
[PORTER_LOG] FileBody_61
[PORTER_LOG] Appears there was a file, continuing
[PORTER_LOG] appending sub param
[PORTER_LOG] filename
[PORTER_LOG] %2FUsers%2Fa%2FDesktop%2Fimages%2Fbutton_send_message.gif
[PORTER_LOG] appending sub param
[PORTER_LOG] content_type
[PORTER_LOG] application%2Foctet-stream%3B+charset%3DISO-8859-1
[PORTER_LOG] appending sub param
[PORTER_LOG] path
[PORTER_LOG] %2Ftmp%2FapreqSmt874
[PORTER_LOG] Signing Filename
[PORTER_LOG] {SHA}s1z1XzoFa8FFhrdkxAWx1CPi2Cc=
[PORTER_LOG] appending sub param
[PORTER_LOG] signature
[PORTER_LOG] s1z1XzoFa8FFhrdkxAWx1CPi2Cc%3D
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] FilePath_61
[PORTER_LOG] FilePath_61=button_send_message.gif&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] RealFilePath_61
[PORTER_LOG] RealFilePath_61=%2FUsers%2Fa%2FDesktop%2Fimages%2Fbutton_send_message.gif&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] FileSize_61
[PORTER_LOG] FileSize_61=1874&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] FileLastModified_61
[PORTER_LOG] FileLastModified_61=Feb+18%2C+2010+10%3A35%3A38+AM&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] FileIndex_61
[PORTER_LOG] FileIndex_61=61&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] FileDescription_61
[PORTER_LOG] FileDescription_61=null&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] ThumbnailRotateAngle_61
[PORTER_LOG] ThumbnailRotateAngle_61=0&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] ThumbnailCropX_61
[PORTER_LOG] ThumbnailCropX_61=0&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] ThumbnailCropY_61
[PORTER_LOG] ThumbnailCropY_61=0&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] ThumbnailCropWidth_61
[PORTER_LOG] ThumbnailCropWidth_61=0&
[PORTER_LOG] Writing plain parameter
[PORTER_LOG] ThumbnailCropHeight_61
[PORTER_LOG] ThumbnailCropHeight_61=0&

However, Rails does not receive the params hashes (there is a FileBody_xx hash up to 55, then there are no more.

One possible clue is that there are many extra post variables which I believe come from the Java applet ("FileSize_32"=>"768", "FileDescription_18"=>"null", "ThumbnailCropHeight_30"=>"0", etc.). Could mod_porter not be allocating enough space for all of the variables?

Thanks for your ideas!